Tuneable oscillatory circuits are usually constructed as LC oscillators in integrated circuit technology. In this context, inductors and capacitors are the frequency-determining components. A controllable capacitor can easily be used to tune the frequency with integrated circuit technology. Such controllable capacitors are implemented, for example, using varactor diodes whose depletion layer capacitance varies with the cut-off voltage. In order to be able to switch the frequency in discrete steps or provide a means of switching over the band of a tuneable oscillator, it is advantageous to introduce not only capacitors which can be tuned in an analog fashion into the LC oscillatory circuit but also capacitors which are embodied so as to be capable of being connected into the circuit and disconnected therefrom.
In the case of VCO capacitors which can be connected into the circuit and disconnected from it in a digital fashion it is normally desirable for the capacitor to have a very high quality level when it is connected into the circuit, but for the parasitic capacitance components to remain small when it is disconnected from the circuit. In addition there is the requirement for parasitic interference influences in the supply voltage to have no influence, or the smallest possible influence, on the capacitance. When a switchable capacitance is used in an oscillator it is also desirable to be able to cover the largest possible frequency range.
FIG. 2 shows by way of example the circuit design of a symmetrically embodied LC oscillator. The oscillator is embodied as a voltage-controlled oscillator VCO. In the VCO shown, an inductor 1, 2 is provided which is connected between two circuit nodes A, B of the VCO and has a center tap. The center tap of the inductor 1, 2 with the inductance value L is connected to a supply potential terminal 4 via a power source 3. Furthermore, a variable capacitor C, which can be adjusted both in an analog fashion and also in discrete steps, is connected between the circuit nodes A, B. The oscillation frequency fOSZ of the oscillator is calculated in a good approximation by the resonance frequency of the LC circuit according to the rule
      f    OSZ    =      1          2      ⁢      π      ⁢                        L          ·          C                    where L designates the overall inductance value and C the overall capacitance value in the oscillatory circuit.
In order to tune in an analogous fashion, an analog tuning signal in the form of a tuning voltage can be fed to an input 6. Furthermore, a digital frequency word of the word length n can be fed to a further control input 7 in order to preselect the frequency range. This defines how many capacitors, acting in parallel as fixed-value capacitors, are connected to the oscillatory circuit.
An oscillation compensation amplifier which comprises two cross-coupled MOS transistors 8, 9 is connected between the circuit nodes A, B. The cross-coupled transistors 8, 9 are connected to reference potential 10. The oscillatory circuit frequency is determined by the inductance value L and the capacitance value C. The amplifier 8, 9 provides a negative impedance which compensates the attenuation of the oscillatory circuit. The output signal with the desired frequency can be tapped at the circuit nodes A, B in differential form. At said nodes, two signals which are phase-shifted by 180 degrees and which oscillate with the frequency fOSZ are made available.
As already explained, the analog change in capacitance is preferably implemented with integrated varactor diodes. In contrast, the frequency tuning in discrete steps can be carried out by means of series switches, as shown in FIGS. 3a to 3c. 
For example, FIG. 3a shows a digitally switchable capacitor 11 which is connected by means of, in each case, one series switch 12, 13 between the circuit nodes A, B in FIG. 2. By simultaneously opening and closing the switches 12, 13, the capacitor 11 either makes a contribution to the determination of the frequency of the oscillatory circuit, or does not.
A different possible way of arranging the series switches is shown by FIG. 3b. In said figure, two capacitors 14, 15 are provided and these each have twice the capacitance value of the capacitor 11 in order to bring about the same change in frequency. The capacitors 14, 15 are each connected by one terminal to the reference potential terminal 10, and by a further terminal to, in each case, one of the switching nodes A, B via, in each case, one series switch 16, 17.
FIG. 3c shows a modification of the circuit from FIG. 3b in which the capacitors 14, 15 and the series switches 16, 17 are interchanged.
All three prior art solutions according to FIGS. 3a to 3c have in common the fact that one or more electronic switches are connected into the circuit in series with a capacitor. This makes it possible to connect a capacitance value into the circuit between the terminals A and B of the VCO and, in turn, to disconnect it from the circuit.
The electronic switches are generally embodied as metal oxide semiconductor transistors which are operated in a switching fashion. For the series resistance of these transistors to remain low for high quality levels of the capacitor which can be connected into the circuit, the transistors must be configured with a correspondingly large channel width. When the transistors are embodied using integrated circuit technology, they then have a considerable chip area requirement. However, the parasitic capacitances which are unavoidably present when such a transistor is integrated also reach considerable values as the size of the switch increases. A further disadvantage of the parasitic capacitances is the reduction in the achievable capacitance ratio between the switched-on capacitor and the switched-off capacitor. However, the achievable change in frequency, and thus the digital tuning range for a VCO which can be provided is thus also small.